Current white-light-emitting phosphor-conversion LEDs (pc-LEDs) utilize one or more phosphors to partially absorb the blue light emitted by InGaN LED dies in order to convert some of the emitted blue light into a yellow light. The remaining blue light and converted yellow light combine to produce light which is perceived as white. While other phosphors may be used in addition to the yellow-emitting phosphor to modify the spectral output of the pc-LED, the yellow-emitting phosphor remains the central component in white pc-LEDs.
By far the most used yellow-emitting phosphor in white pc-LEDs is a cerium-activated yttrium aluminum garnet, Y3Al5O12:Ce, phosphor (YAG:Ce). An example of a YAG:Ce phosphor and its application in a white pc-LED is described in U.S. Pat. No. 5,998,925. Some composition modifications of YAG:Ce phosphors are also described, such as using Ga to replace Al (YAGaG:Ce) or Gd to replace Y (YGdAG:Ce). Generally, a Ga substitution of Al in YAG:Ce shifts the phosphor's emission peak to shorter wavelengths whereas a Gd substitution of Y shifts the emission peak to longer wavelengths.
One of the challenges in making white pc-LEDs is maintaining color consistency between the white pc-LED packages. A major contributor to this problem and one which is difficult to control from a package manufacturing perspective is related to variations in the dominant emission wavelength (λd) of LED dies. In particular, the blue emission of LED dies can have a large range, e.g., 440 nm to 470 nm, resulting from the semiconductor manufacturing process. This inherent die-to-die variation is accentuated when the LED die is combined with a phosphor in a white pc-LED. If the dominant emission λd of the LED die is close to the absorption maximum of the phosphor (about 460 nm for YAG:Ce and YGdAG:Ce), more blue light will be absorbed and more yellow light emitted. If the dominant emission wavelength λd is further away from the absorption maximum of the phosphor, less blue light is absorbed and less yellow light is emitted resulting in a different white emission color.
One way to limit package-to-package variations in white pc-LEDs is to tightly bin the LED dies, i.e., preselect only LED dies that emit within a narrow range. However, tighter binning increases cost since a smaller percentage of the LED dies from a semiconductor manufacturer are suitable for use. Therefore, it would be an advantage to have a white pc-LED package that was less sensitive to the emission variations of blue LED dies.